Magnetically-and electrically-induced variable resistance materials and method for preparing same

ABSTRACT

Provided are new compositions of ruthenates in the pervoskite and layered pervoskite family, wherein the ruthenate compositions exhibit large magnetoresistance (MR) and electric-pulse-induced resistance (EPIR) switching effects, the latter observable at room temperature. This is the first time large MR and EPIR effects have been shown together in ruthenate compositions. Further provided are methods for synthesizing the class of ruthenates that exhibits such properties, as well as methods of use therefor in electromagnetic devices, thin films, sensors, semiconductors, insulators and the like.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. application Ser. No. 10/390,362filed Mar. 17, 2002, which is incorporated herein in its entirety.

GOVERNMENT INTEREST

This invention was supported in part by Grant Nos. DMR 00-79909 and DMR99-88853 from the National Science Foundation. Accordingly, theGovernment may have certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to a new class of magnetically andelectrically induced variable resistance materials having a largemagnetoresistance effect (MR) and a large electric-pulse-inducedresistance switching effect (EPIR), and a method for preparing saidmaterials.

BACKGROUND OF THE INVENTION

Magnetoresistance (MR) effects are utilized to detect variations withina magnetic field by converting into variations of resistance. Materialsthat exhibit MR effect also often exhibit spin-polarization ability.Therefore, they can form spin valves to transmit signals based on spinpolarization. Recent advances in data storage technologies have beenbased upon the use of multilayer metallic thin films that exhibit giantmagnetoresistance (GMR) (e.g., U.S. Pat. No. 5,977,017 (Golden et al.)).MR-based devices, e.g., magnetoresistors, magnetic heads to detectsignals of a magnetic recorder, or probes of a spin-polarized scanningtype tunneling microscope, have the advantage of high immunity toradiation damage. Thus, data storage is indestructible, even underadverse operating or storage conditions.

Oxides are known to have large MR properties. In a class of oxidematerials called manganates (see below), such large MR has been termedcolossal magnetoresistance (CMR). (Ramirez, J. Phys.: Condens. Matter9:8171-8199 (1997)). CMR is also commonly used to refer to the large MRobserved in similar oxides as manganates when cobalt is used to replacemanganese, called cobatates.

In addition to the large MR effect, oxides are also known to exhibitelectric-pulse induced resistance (EPIR) properties. (Liu et al., Appl.Phys. Lett. 76(19):2740-2752 (2000)). EPIR switching effects areutilized to permanently change resistance, until a reverse electricalpulse is applied. Liu et al. teach a method for switching the propertiesof perovskite materials used in thin film resistors by applying shortelectrical pulses to change (both reversibly and non-reversibly) theelectrical, thermal, mechanical and magnetic properties of the materialwithout damaging it (Liu et al., Appl. Phys. Lett. 76:2749-2751 (1999);U.S. Pat. No. 6,204,139). This permits the formation of memory devicesand resistors in electronic circuits that can be varied in resistance.Materials exhibiting an EPIR effect are useful as non-volatile,rewritable memory cells to store information in microelectronic devices.For example, U.S. Pat. No. 6,473,332 (Ignatiev et al.) teaches anelectrically operated, overwritable, multivalued, non-volatile resistivememory element that includes a two terminal non-volatile memory deviceusing CMR oxide film material and a defined circuit topologicalconfiguration that takes advantage of the variable EPIR effect of thethin film material.

For the purpose of the present application, perovskite compositionsrefer to the formula ABO₃, in which A represents a metal that may bedrawn from alkali, alkali earth, rare earth, or other metal such aspotassium, strontium, lanthanum, neodymium, cerium, yttrium, lead,bismuth or the like, and B represents a transition metal such as cobalt,iron, nickel, or the like. (Perovskite actually can have even broadercompositions for metal substitution, as will become clear later in thefollowing description.) Perovskite-type materials have long been knownto be useful for the catalytic oxidation and reduction reactionsassociated with the control of automotive exhaust emissions (U.S. Pat.No. 4,107,163 (Donohue)), but the electrical conductivitycharacteristics of the material are more recently discovered, such asthe CMR effect observed in manganites, when B represents manganese (Mn),and cobatates, when B represents cobalt (Co) (Ramirez, 1997). Kobayashiet al., also made various studies on other, non-Mn and non-Co, orderedperovskite oxide crystals and identified a MR phenomenon, permitting thedevelopment of a magnetoresistor that is an oxide crystal with anordered double perovskite crystal structure (Kobayashi et al., Nature395:677-680 (1998); U.S. Pat. No. 6,137,395). (See below onSr(Fe_(0.5)Mo_(0.5))O₃ and Sr(Fe_(0.5)Re_(0.5))O₃).

As made clear from above, for the purpose of the present application,manganates refer to a known class of perovskite oxides in which Brepresents manganese (Mn). Many manganates have CMR properties.Consequently, upon application of a magnetic field, the electricalresistivity of the material drops drastically due to a field-inducedswitching of the crystal structure. However, manganate MR is limited toa certain temperature range in which the magnetic field can promote (orin some compounds induce) a phase transition. The resistance response tothe magnetic field is often hysteretic, in that a different resistanceis found at the same magnetic field depending on the history of thefield, such as whether the field has been increasing or decreasing. Inaddition, there is significant 1/f noise associated with circuitscomprising manganate MR materials. It is generally accepted that inperovskite compositions, manganese may be held in both the trivalentMn³⁺ state and the tetravalent Mn⁴⁺ state, and that such mixed valencyis an essential element for the phase transition and the resultant largeMR properties of the compound. Very similar mixed valency also exists incobaltates that exhibit CMR effect.

Some manganates and cobaltates exhibit both CMR and EPIR effects.Generally, most EPIR effects were observed in thin films, across which alarge electrical field may be achieved using a relatively modestelectrical voltage. For example, Liu et al. describes such effect in CMR(Pr_(0.7)Ca_(0.3))MnO₃ thin films. (Liu, 2000). In the patent of Liu etal. (U.S. Pat. No. 6,204,139), an additional cobaltate of a metalcomposition Gd_(0.7)Ca_(0.3)BaCO₂ was described as having EPIRproperties. According to Liu et al (U.S. Pat. No. 6,204,139 B1), otheroxide materials of the perovskite-related families that exhibit EPIReffect include YBa₂Cu₃O₇ which is also known to possess high Tcsuperconductivity.

Mixed valency also appears to be important for the EPIR effect. Themanganate that is best known for having the EPIR effect,(Pr_(0.7)Ca_(0.3))MnO₃, contains 70% Mn³⁺ and 30% Mn⁴⁺. Bothcobaltateand the high Tc superconductor described above are known tohave mixed valency. However, at least in the case of manganates, adisadvantage of such mixed valency is that manganates are sensitive tosurface conditions. As a result, Mn ions at a surface that is exposed toair, vacuum, or other atmospheric or liquid environment, or at theinterface that is adjacent to another solid material, can have differentdegrees of mixed valency distinct from those in the interior, dependingon the moisture and chemical nature of the environment. As a result ofsuch sensitivity, the magnetic (MR) and electrical (EPIR) responses ofthe resistivity are sometimes difficult to control or predict. Inaddition, mixed valency is sensitive to radiation, laser illumination,and other non-magnetic and non-electrical stimuli, which could alsoaffect the magnitude of the observed MR and EPIR effect.

Several other families of oxides are also known to exhibit large MReffects, or large EPIR effects, but not both. One family of oxides thathave been known to have large MR properties is based on the compositionSr(Fe_(0.5)Mo_(0.5))O₃ mentioned previously, which also has aperovskite-type structure ABO₃. In this case, however, B is comprised,in equal parts, of Fe and Mo, which are ordered. Thus Fe and Mo arelocated on alternating sites in a checkerboard type of arrangement. TheMR capability of this material results from a tunneling effect acrossgrain boundaries. Therefore, substantial magnetoresistance is lost whenSr(Fe_(0.5)Mo_(0.5))O₃ is formed into a single crystal or an epitaxialfilm. It is believed that a 1:1 ordered cation arrangement is importantfor its magnetoresistance properties. However, disadvantageously, Mo inthe above compound has a valence state of Mo⁵⁺, as opposed to the morecommon Mo⁶⁺ state, therefore a careful preparation under a reducingatmosphere or vacuum is required to form such compounds. Another orderedperovskite, Sr(Fe_(0.5)Re_(0.5))O₃, reportedly exhibits similar MRproperties, and suffers from the same limitations. These compounds arenot known to exhibit EPIR effect.

By comparison, Beck et al., have claimed a large number of oxidefamilies that reportedly exhibit EPIR effect (PCT application,PCT/IB00/0043)). However, the examples in the PCT application andpublications by the same group of researchers at IBM Zurich reveal thatonly the following oxides demonstrated the claimed EPIR effect: dopedBa_(1-x)Sr_(x)TiO₃, when doped with p-type (chromium or manganese) orn-type (vanadium or niobium) dopants; chromium-doped SrZrO₃, dopedCa₂Nb₂O₇ and doped Ta₂O₅ with chromium or vanadium as dopants (see, Becket al., Appl. Phys. Lett. 77(1):139-141 (2000); Watanabe et al., Appl.Phys. Lett. 78(23):3738-3740 (2001); Rossel et al., J. Appl. Phys.90(6):2892-2898 (2001)). Earlier reports also indicated that Al₂O₃,Nb₂O₅, TiO₂, Ta₂O₅ and NiO may exhibit memory behavior based oncurrent-induced bistable resistance switching or voltage-controllednegative resistance phenomena (see references cited by Beck et al.,2000). None of the cited materials, however, exhibit a large MR effect.

Many ruthenate oxides, on the other hand, are known to be excellentconductors. For example, strontium ruthenate (SrRuO₃), which is also aperovskite, is often used as a bottom electrode material in electronicdevices (Eom et al., Science 258:1766-1769 (1992); Eom et al., Appl.Phys. Lett. 63:2570-2572 (1993); Tiwari et al., Appl. Phys. Lett.64:634-636 (1994); Klein et al., J. Magn. Mater. 188:319-325 (1998);FIG. 1 of PCT/IB00/00043). SrRuO₃ itself is metallic and ferromagnetic(Longo et al., J. Appl. Phys. 39:1327-1328 (1968)), which is uniqueamong 4d transition metal oxides. It has a very small (0.5-4%) MR,restricted to a narrow temperature range near theferromagnetic/paramagnetic transition temperature. When SrRuO₃electrodes were used in place of the Pt layer in Pb(Zr,Ti)O₃-basedferroelectric memory devices, they alleviated the problem ofpolarization fatigue (Eom et al., 1993). It is, however, not known tohave EPIR effect.

The ABO₃ perovskite structure of SrRuO₃ also makes it compatible withother similar compounds, allowing its incorporation as an epitaxial thinfilm or buffer layer in heteroepitaxial device structures built onperovskite oxides (Tiwari et al., 1994). Further, substitution of Ru byother transition metal ions (Mn, Fe, Co, etc.) has been reported tocreate new magnetic properties, ranging from colossal magnetoresistance(e.g., in Sm, Ca(Ru,Mn)O₃, with Ru in amount considerably less than Mn)(Raveau et al., J. Supercond. 14:217-229 (2001)) to spin glass behavior(e.g., in Sr(Fe_(0.5)Ru_(0.5))O₃) (Battle et al., J Solid State Chem.78:281-293 (1989)). Related Ru-based compounds further display anexceptionally rich variety of electronic and magnetic properties,ranging from paramagnetic (e.g., in CaRuO₃) to superconducting (e.g., inSr₂RuO₄) while retaining metallicity (Longo et al., 1968; Maeno et al.,Nature 372:532-534 (1994)). None of these Ru-based compounds ortransition-metal substituted ruthnates are known to have EPIR effect.

Another ruthenate composition, TlSrRuO (Tl:Sr:Ru at a ratio of 1:2:1),which is believed to have a layered structure that is derived fromperovskite, were reported to have magnetic transitions (e.g., U.S. Pat.No. 5,759,434 (Shimakawa et al.)). As a result, magnetoresistance in thevicinity of the transitions have been observed. This material is highlytoxic, however, because of the presence of Tl. It is also not known tohave EPIR effect.

A review of the ruthenate literature reveals that no EPIR effect hasbeen reported, but considerable difficulties have been encountered insynthesizing these compounds. Consequently, highly uniform materialsfree of second phases or heterogeneous clusters are difficult to obtainusing standard solid state reactions of mixed starting oxide powders.Often, very long calcination and/or laborious regrinding and remixing isneeded, especially when two B-site cations are desired in the resultingcompound (Battle et al., 1989; Kim et al., J. Solid State Chem.114:174-183 (1995); He et al., Phys. Rev. B63:172403 (2001)). It is alsoknown that even minor second phases and small heterogeneous clusters canhave a considerable effect on the magnetic and electrical properties ofthese compounds (Kim et al., 1995; He et al., 2001).

Accordingly, there has, until the present invention, existed a need forcommercially useful material exhibiting both MR and EPIR properties,that is non-toxic and easily integrable with electrode and othermaterials for device applications, without suffering from drawbacks,such as sensitivity to surface environment. High purity ruthenateperovskites might satisfy that need if they, and their relatedcompounds, were not so difficult to form by conventional ceramicprocessing routes using mixed starting oxide powders and solid statereactions. Thus, there has been a further need, until the presentinvention, for a production method for synthesizing chemically uniformruthenates and doped ruthenates that exhibit both large MR and EPIReffects, wherein the method significantly reduces the processing timeand results in excellent compositional uniformity, as verified bydiffraction and magnetic measurements.

SUMMARY OF THE INVENTION

The present invention teaches new compositions of ruthenates thatexhibit large magnetoresistance (MR) and electric-pulse-inducedresistance (EPIR) switching effects, the latter observable at roomtemperature. This is the first time that both large MR and EPIR effectshave been reported together in ruthenates in the pervoskite- and layeredpervoskite-related family of compounds. Previously, no compositionhaving a majority of ruthenium (no less than 50% in B) were known tohave large MR. The present invention, therefore, also teaches methodsfor synthesizing this large class of ruthenates in theperovskite-related family that exhibits such properties. Since theapplications of both MR and EPIR require the use of an electrode,whereas SrRuO₃ and several other ruthenates are already recognized asexcellent electrode materials, ruthenate electrodes and MR orEPIR-active ruthenate films or devices can be advantageouslyconstructed. These constructions have the additional advantage ofstructural and chemical compatibility. At ambient conditions, ruthenatesare also chemically stable and do not suffer from varied valence statesaccording to the surface environment.

It is, therefore, an object of the invention to provide a nontoxic,magnetically and electrically induced variable resistance compositioncomprising a ruthenate formulation of the perovskite family ofmaterials, wherein the ruthenate exhibits large MR. It is a furtherobject to provide a nontoxic, magnetically and electrically inducedvariable resistance composition comprising a ruthenate formulation ofthe perovskite family of materials, wherein the ruthenate exhibits anEPIR switching effect. Moreover, it is a preferred object to provide anontoxic, magnetically and electrically induced variable resistancecomposition comprising a ruthenate formulation of the perovskite familyof materials, wherein the ruthenate exhibits both large MR and an EPIRswitching effect. The large MR effect occurs over a broad range oftemperatures.

It is also an object to provide compositions comprising an oxideformulation represented by A(Ru_(1-x)M_(x))O₃, where 0<x<1, M isselected from the group consisting of magnetic elements; A is selectedfrom the group consisting of K, Ca, Sr, Ba, Pb, Bi, La, Ce, Pr, Nd, Pm,Sm, Eu, Gd, Tb and Y, as well as mixtures thereof. Preferably,0.5<x<0.9; more preferably 0.01<x<0.5. In an alternative embodiment ofthe foregoing, elements such as Sc, Y, Zr, Nb, Hf, Ta, Al, Ga, Ge and Snare incorporated into M in the minority to vary the magnitude ofelectrical resistance. Moreover, in preferred embodiments the magneticelements M are selected from the group consisting of Ti, V, Cr, Mn, Fe,Co and Ni, as well as any mixture thereof.

It is another object to provide compositions comprising a spin glassformulation represented by A_(n+1)(Ru_(1-x)M_(x))_(n)O_(3n+1), where nis any positive integer, 0<x<1, M is selected from the group consistingof magnetic elements; A is selected from the group consisting of K, Ca,Sr, Ba, Pb, Bi, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb and Y, as well as anymixture thereof. Preferably, 0.5<x<0.9; more preferably 0.01<x<0.5. Itis also an object to provide compositions having the formulaA₂(Ru_(1-x)M_(x))O₄. In alternative embodiments of the foregoing,minority elements, such as Sc, Y, Zr, Nb, Hf, Ta, Al, Ga, Ge and Sn areincorporated into M in minority to vary the magnitude of electricalresistance. As indicated in the formulas described above, in preferredembodiments the magnetic elements M are selected from the groupconsisting of Ti, V, Cr, Mn, Fe, Co and Ni, as well as any mixturethereof.

Ruthenates of the above compositions, and their structural families, areadvantageously used in accordance with the invention in microelectronicor optical devices, such as, but without limitation, magnetic sensors, athin film between metals, or semiconductors, or insulators, or oxides,or any combination of the above, which take advantage of, or benefitfrom, the MR or EPIR properties of the provided ruthenates.

Compared to solid state reactions using mixed starting oxides, thesolution-polymerization method significantly decreases the processingtime and improves the compositional uniformity of the ruthenatecompounds. The method especially offers a clear advantage in processingdoped ruthenate compounds, allowing magnetic properties of newruthenates to be sensitively studied without the complication ofimpurity phases or inhomogeneous clusters. Accordingly, it is a furtherobject of the invention to provide a method of preparing a nontoxic,magnetically and electrically induced variable resistance ruthenatecompositions of the types described above, comprising preparing aruthenate solution and processing said solution by a sol gelsolution-polymerization method, such that there is a uniform andhomogeneous distribution of source cations that were chelated in thesubsequently polymerized precursor. The composition is preferably of maybe of polycrystalline or mixed ruthenate form, wherein thepolycrystalline form is of the formula SrRuO₃, and wherein the startingmaterial comprises RuO₂.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description, examples and figures whichfollow, all of which are intended to be for illustrative purposes only,and not intended in any way to limit the invention, and in part willbecome apparent to those skilled in the art on examination of thefollowing, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe invention, will be better understood when read in conjunction withthe appended drawings. It should be understood, however, that theinvention is not limited to the precise arrangements andinstrumentalities shown.

FIG. 1 is a graphical illustration of an exemplary material,Sr_(1-x)La_(x)Ru_(1-x)Fe_(x)O₃, in accordance with a preferredembodiment, at 10 K as a function of applied magnetic field fordifferent substitution values.

FIG. 2 is a graphical illustration of the magnetoresistance at 10 K as afunction of applied magnetic field for other exemplary materials.

FIG. 3 is a graphical illustration of the switching behavior of a thinfilm of an exemplary material, Sr_(0.7)La_(0.3)Ru_(0.7)Fe_(0.3)O₃,prepared on a silicon substrate with Pt coated Si as the bottomelectrode upon application of voltage pulses of amplitude 10 V and 600ns duration.

FIG. 4 is a graphical illustration showing the ability to repeatedlychange resistance of an exemplary material,Sr_(0.7)La_(0.3)Ru_(0.7)Fe_(0.3)O₃, prepared on a silicon substrate withPt-coated silicon as the bottom electrode by the application of a single10V electrical pulse of alternating polarity and duration.

FIG. 5 depicts room temperature powder x ray diffraction (XRD) patternsof an exemplary material, Sr_(1-x)La_(x)Ru_(1-x)Fe_(x)O₃ taken with CuKαradiation and indexed using a pseudocubic cell of perovskite. There isno evidence of impurity phases near the 110 reflection.

FIG. 6 is a magnetic phase diagram of an exemplary material,Sr_(1-x)La_(x)Ru_(1-x)Fe_(x)O₃, determined using ac/dc magnetizationdata. Ferromagnetic (FM); Paramagnetic (PM); Spin glass (SG). The insetshows the temperature dependence of the ac susceptibility underfield-cooled (100 Oe) and zero-field cooled conditions.

FIG. 7 shows field dependence of the MR of an exemplary material,Sr_(1-x)La_(x)Ru_(1-x)Fe_(x)O₃, for x=0.1-0.3 at 10 K. The MR isnormalized by the resistance at zero field. The inset shows the MR has amaximum at x=0.3 at both 10 and 30 K, and that the MR at 10 K is larger.At 10 K, the resistivity of x=0.4 was too high to measure in this study.All data are from bulk polycrystals except the ones shown by the curveof connecting open circles that were from a (001) thin film with x=0.3.

FIG. 8 shows field dependence of the MR of an exemplary material,Ca_(1-x)La_(x)Ru_(1-x)Fe_(x)O₃, for x=0.2-0.3 at 30 K. The MR isnormalized by the resistance at zero field. The inset shows the magneticphase diagram.

FIG. 9 shows field dependence of the MR of an exemplary material,Sr_(1.8)La_(0.2)Ru_(0.8)Fe_(0.2)O₄. The MR is normalized by theresistance at zero field. The inset shows the temperature dependence ofthe ac susceptibility under field-cooled (100 Oe) and zero-field-cooledconditions, and indicates hysteresis below the freezing temperature.

FIG. 10 graphically displays a XRD measurement of an exemplary material,SrRuO₃, as a function of calcination temperature and time in the ceramicroute. * marks RuO₂ and SrO; o marks traces of second phases. Perovskitereflections are indexed in pseudocubic notations.

FIG. 11 graphically displays an XRD measurement of an exemplarymaterial, SrRuO₃, prepared by the solution-polymerization Process (A)and Process (B). * marks RuO₂ and SrO; K_(β) marks 110 reflection fromthe CuK_(β) line.

FIG. 12 graphically displays an XRD measurement of an exemplarymaterial, Sr_(0.7)La_(0.3)Ru_(0.7)Fe_(0.3)O₃, prepared by the ceramicroute. * marks RuO₂, La₂O₃ and SrO; o marks traces of second phase;K_(β) marks 110 reflection from the CuK_(β) line.

FIG. 13 graphically displays an XRD measurement of an exemplarymaterial, Sr_(0.7)La_(0.3)Ru_(0.7)Fe_(0.3)O₃, prepared bysolution-polymerization Process (B). * marks RuO₂ and SrO; K_(β) marks110 reflection from the CuK_(β) line.

FIG. 14 graphically displays DC magnetization at 0.1 T (tesla) as afunction of temperature of an exemplary material,Sr_(0.7)La_(0.3)Ru_(0.7)Fe_(0.3)O₃, prepared by the ceramic route andthe solution-polymerization process (B), respectively. Inset shows ACsusceptibility with a peak at the T_(c) of SrRuO₃ clusters.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In accordance with a preferred embodiment, the present inventionincludes a family of ruthenates, characterized by the perovskite oxideformula A(Ru_(1-x)M_(x))O₃, that when synthesized exhibit (i) a large MReffect over a broad range of temperatures, and (ii) an electrical EPIReffect at room temperature. In the formula A(Ru_(1-x)M_(x))O₃, x ispreferably greater than 0 and smaller than 1; M is preferably chosenfrom the group consisting of the following magnetic elements: Ti, V, Cr,Mn, Fe, Co, and Ni (or any mixture thereof), and A is preferably chosenfrom the following: K, Ca, Sr, Ba, Pb, Bi, La, Ce, Pr, Nd, Pm, Sm, Eu,Gd, Tb and Y (or any mixture thereof). Preferably, 0.1<x<0.9; morepreferably 0.01≦x≦0.5. Optionally, other elements, including Sc, Y, Zr,Nb, Hf, Ta, Al, Ga, Ge, and Sn, may also be incorporated as M inminority to vary the magnitude of electrical resistance.

By ‘mixture’ is meant the combination of two or more compounds selectedfrom the foregoing lists using recognized methods. For example, eitherCa or Sr may be used individually in the formulation at position A, or amixture of any number of the compositions listed for the A position maybe mixed, such as a combination of Ca and Sr together as a mixture,which mixture may then instead be used in the formula at position A.Similarly, any number of the compositions listed for position M may alsobe mixed to fill the M position in the formulation. Thus, two or more ofthe various components of the starting materials can be combined topre-react, for example, before finally combining together in the finalformulation. ‘Mixture’ in this situation, however, does not usuallyrefer to the mixing of the selected compositions listed for position Awith those listed for position M in the formulation. Some elements, suchas Y and Tb, may naturally enter both position A and position M.However, these are exceptions and not rule.

The source or starting materials for the formulation can be selectedfrom purified forms of the metals, salts or other forms, because once ithas been added to the formula for the final perovskite product, eachcomponent exists as an ion. For example, Ru metal may be used to replaceRuO₂ as a starting material.

In addition to the oxides mentioned above, which are all perovskites orrelated to perovskites having structures consisting of a network of BO₆octahedra, where B is a metal cation, where B includes both Ru and M, inaccordance with an alternative embodiment, the present inventionincludes an additional class of octahedral network materials thatcontains mostly BO₆, although such network is grouped into layers. Onesuch family of layered perovskite materials is represented byA₂(Ru_(1-x)M_(x))O₄, in which x is preferably greater than 0 and smallerthan 1, where M is preferably chosen from the group consisting of thefollowing magnetic elements, Ti, V, Cr, Mn, Fe, Co, and Ni (or itsmixture), and A is preferably chosen from the group consisting of: K,Ca, Sr, Ba, Pb, Bi, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb and Y (or itsmixture). Optionally, other elements, including Sc, Y, Zr, Nb, Hf, Ta,Al, Ga, Ge, and Sn, may also be incorporated as M in minority to varythe magnitude of electrical resistance.

The present invention also includes other layered perovskite ruthenatesof the formula A_(n+1)(Ru_(1-x)M_(x))_(n)O_(3n+1), where n is anypositive integer, and A and M are similarly comprised as describedabove. Thus, A₂(Ru_(1-x)M_(x))O₄, mentioned above, belongs to thisfamily in the special case of n equal to one. Large MR effects that areenhanced at low temperatures and large EPIR effects at room temperaturecan be expected in these materials.

The thus-prepared Ru-rich perovskites and layered perovskites,containing magnetic elements, have large electrical resistance changeunder magnetic stimulation at low temperatures (the MR effect). They canalso be prepared in air without difficulty, unlike some of the othermagnetoresistant oxides for which a deliberate preparation under areducing atmosphere or vacuum is needed. At room temperature, they canhave large electrical resistance change under the stimulation of a largeelectrical field (the EPIR effect), which may be caused by a smallvoltage when the materials are in the form of a thin film.

Although Ru has been used as an additive to manganates to alter theirproperties in the prior art, Ru has not been the primary component ofthe composition. Although mixed valency may occur in Ru in certaincompositions of the foregoing (for example when Co is present), thepresence of the large MR and EPIR in the preferred embodiments of theinvention is not sensitive to whether Ru has mixed-valency or not.

In accordance with the present invention, any known chemical method ofmixing elements and forming powders or ceramics may be used to form thebulk materials, such as the sol gel methods, or standard ceramicprocessing methods, e.g., metallo-organic decomposition methods, orchemical vapor deposition methods. A preferred synthetic method toprepare the ruthenate materials of the present invention is the sol gelmethod as described in Examples that follow.

However, the ruthenate materials can also be synthesized by standardceramic processing methods as used to form bulk materials. For example,to obtain (Sr_(0.7)La_(0.3))(Ru_(0.7)Fe_(0.3))O₃, starting materialssuch as SrCO₃, La₂O₃, RuO₂, and Fe₂O₃ powders were first mixed usingstandard mixing techniques, such as hand mortar mixing, ball milling, orattrition milling, then fired at various temperatures for various timesfrom 750° C. to 1200° C. to obtain a uniform powder of the(Sr_(0.7)La_(0.3))(Ru_(0.7)Fe_(0.3))O₃ composition. The powder waspreferably verified, for example, using powder x-ray diffraction (XRD).Based upon the diffraction pattern, the resulting material wasclassified as an orthorhombic type of perovskite. The powder was thencompacted and sintered to high density to produce the resultingmaterial, commonly referred to as ceramic, for magnetic and electricalapplications. However, the presence of other minority phases in thecomposition has no significant effect on the resulting large MR and EPIReffects. In fact, the existence of selected minority phases in thecomposition may be advantageous under certain conditions.

Common phases that may coexist with the above perovskite include, forexample, SrRuO₃ which has a ferromagnetic transition, various layeredperovskites that have a higher ratio of (Ru, M) to A than one, as in theform of A(Ru_(1-x)M_(x))O₃, (the ratio of B to A is one, if B is viewedas (Ru_(1-x)M_(x))), and other forms of ABO₃ compounds whose structuresare not perovskite-based, and RuO₂. The small modification of structure,in the lattice parameter or in the unit cell distortion, which may beintroduced by using different elements among M and different elementsamong A, may also be practiced. This can be advantageous, for example,in preparing thin films when the lattice matching of the film and thesubstrate crystals is important for maintaining the film quality.Likewise, when ruthenates of various layered perovskite structures areprepared, for example (Sr_(1.8)La_(0.2))(Ru_(0.8)Fe_(0.2))O₄, of theA₂(Ru_(1-x)M_(x))O₄ family, common phases that may coexist withoutdeleterious effect on MR and EPIR effects, include, for example, SrRuO₃,as well as other forms of ABO₃ compounds whose structures are eitherperovskite or not perovskite-based, or various other layered perovskitesthat have a ratio other than 2 of A to (Ru, M) as inA₂(Ru_(1-x)M_(x))O₄, or RuO₂.

The magnetoresistance of the material is preferably verified bymeasurement of electrical resistance as a function of magnetic field.Typically, this is performed by using the “four-point-probe” method, andelectrical resistance decreases with the magnetic field. A dependence ofthe electrical resistance with the magnetic field is shown for threecompositions, (Sr_(0.7)La_(0.3))(Ru_(0.7)Fe_(0.3))O₃,(Sr_(0.8)La_(0.2))(Ru_(0.8)Fe_(0.2))O₃, and(Sr_(0.9)La_(0.1))(Ru_(0.9)Fe_(0.1))O₃, in FIG. 1. The percentage ofresistance change increases with decreasing temperature for all threecompositions. Examples of other compositions include replacing Fe ingroup M by one or more of other elements from the group of Ti, V, Cr,Mn, Co and Ni, and replacing A by one or more of other elements from thegroup of K, Ca, Sr, Ba, Pb, Bi, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb andY. FIG. 2 illustrates the effect of replacing Fe by Co, replacing Sr byCa, and replacing La by Pr, respectively. Nevertheless, as shown in FIG.2, large MR is observed in all cases. Most likely at least some Cr, Mn,Fe, Co, or Ni is needed for a MR effect, although for EPIR, therestriction are less stringent.

The materials prepared in accordance with a preferred embodiment of theinvention may also be used in thin film form. For example, it may beused to prepare a magnetic sensor or a nonvolatile resistive memoryelement. Ceramic ruthenates may also be used as a target to form thinfilms on a selected substrate using standard deposition techniques, suchas, but without limitation, multi-target pulse laser deposition PLD(with targets of individual oxides), DC and radiofrequency (RF)sputtering, magnetron sputtering, ion beam sputter deposition, andreactive physical vapor deposition (these methods require a ceramictarget). Other standard thin film deposition techniques that may or maynot require a ceramic target, such as, but without limitation,metal-organic CVD, reactive physical vapor deposition, slurry coating,electrophoretic deposition and sintering, high velocity flame spraying,as well as sol-gel spin coating process, or any mixture of the describedmethods, may also be employed, as are standard deposition methods thatuse metal (rather than ceramic) targets. In one preferred embodiment,thin films are prepared using the sputtering technique. In another,sol-gel processes are preferred. In yet another (the PLD technique),targets made of material of the required composition, prepared by thesol-gel process or standard ceramic process, are ablated by an excimerUV laser to produce a thin film material of the same composition on thesupplied substrate. Depending on the substrate type and temperatureduring deposition, polycrystalline or single crystal thin films can beprepared.

The present invention also provides a method for preparing a thin filmmagnetic sensor formed on a substrate. The thin film magnetic sensor ismade of a magnetoresistive material prepared in the manner describedabove, with electrical contacts arranged in the four-pointconfiguration. By choosing the appropriate composition, the electricalresistance that decreases nearly linearly with the applied field, up toat least 9 T, can be obtained. Epitaxial and single crystal thin filmsare often obtained when single crystals are used as substrates. However,commonly, such films have the same MR effect as in the bulk ceramics,whether the films are epitaxial or not (see FIG. 7 in Example 1 thatfollows for a composition of Sr_(0.7)La_(0.3)Ru_(0.7)Fe_(0.3)O₃ on asingle crystal oxide substrate). Such a sensor can be advantageouslyused at low temperature under a high field, for example, in thesuperconducting coil of a MRI device commonly utilized for medicalimaging.

The materials in accordance with these embodiments exhibit EPIR effectin the electrical properties, as shown in FIG. 3. This illustrates theswitching behavior of the thin film of the compositionSr_(0.7)La_(0.3)Ru_(0.7)Fe_(0.3)O₃, prepared on a silicon substrate withPt as the bottom electrode. With the application voltage pulses ofamplitude 10 V and 600 nanoseconds (ns) in duration, the resistance ofthe film is increased or decreased depending on the pulse polarity. FIG.4 illustrates the ability of the material to repeatedly changeresistance by the application of a single electrical pulse ofalternating polarity. The magnitude of the change can be controlled bythe pulse width and voltage. Such a thin film provides a means to storeinformation in microelectronic devices in a simple two terminalconfiguration. The memory is non-volatile, multivalued, andoverwritable.

Composition and film thickness can also be adjusted to obtain differentswitching behavior. For example, with decreasing Fe and La content(i.e., decreasing x) in the Sr_(x)La_(1-x)Ru_(1-x)Fe_(x)O₃ series, theresistance decreases. Conversely, by increasing Ti content (i.e.,increasing x) in SrRu_(1-x)Ti_(x)O₃, or by increasing Zr in SrRu_(1-x)Zr_(x)O₃, the resistance increases. These adjustments will allowthe memory cell resistance, which depends on the cell size and filmthickness, e.g., to fall in a range that is suitable for microelectronicdevice applications.

Compared to the manganate materials taught by the prior art, ruthenatesgenerally have a lower deposition temperature allowing highlycrystalline thin films to form. The lower crystallization temperaturemakes the disclosed materials better candidates for integration withexisting silicon-based electronics technology.

EXAMPLES Example 1 Large Negative MR and EPIR in B-Site-Fe-DopedRuthenates

Sr_(1-x)La_(x)Ru_(1-x)Fe_(x)O₃ series—Magnetically glassy Fe-dopedruthenates of perovskite and layered perovskite can be derived from aparent compound that is either magnetic or nonmagnetic. In this example,the materials were obtained by alloying FeO₆ octahedra into parentruthenate compounds that are metallic. These parent compounds are eitherferromagnetic (SrRuO₃) or paramagnetic (CaRuO₃ and Sr₂RuO₄) and theirstructures are either perovskite type (SrRuO₃ and CaRuO₃) or layeredperovskite type (Sr₂RuO₄). Since octahedral Fe³⁺ ions in perovskite-typestructures are known to couple antiferromagnetically, magneticfrustration easily arises in a randomly Fe-alloyed BO₆ network,resulting in a spin glass. The resulting large negative MR that isobserved appears to be associated with the Fe impurities which operateas atomic-scale “spin valves,” allowing electrical current to passthrough when the spatially adjacent electronic states are spin aligned.

Using a modified sol-gel method the materials were synthesized to firstobtain mixed ruthenate powders of high re-activity. Typical powdersafter calcination had a Brunauer-Emmett-Teller (B.E.T.) surface area of16 m²/g, indicating a particle size of about 30 nm. Sintering wasconducted in air at a temperature ranging from 1200 to 1450° C. usingSrRuO₃ powder packs to suppress Ru volatilization. The XRD patterns ofthree sintered compounds are shown in FIG. 5. Thermogravimetrymeasurements found no weight change up to 800° C., indicating thestability of Ru⁴⁺ and Fe³⁺ valence states, since any mixed valence wouldhave caused oxygen exchange starting at 400-600° C. when oxygendiffusion becomes viable. Near (x-ray) absorption edge fine structuresalso verified the above valence states (data not shown).

Magnetic characterization was performed using a magnetometer (QuantumDesign PPMS) under ac and dc magnetization conditions. Upon Fe doping,magnetic frustration eventually manifests itself in the spin glassstate, which is outlined in the phase diagram in FIG. 6 for theSr_(1-x)La_(x)Ru_(1-x)Fe_(x)O₃ series. In this diagram, the T_(c) of theparamagnetic-to-ferromagnetic transition decreases with x until theferromagnetic phase disappears at x<0.3. Representative spin glassbehavior (shown in the inset of FIG. 6) of hysteretic magnetizationbelow the freezing temperature is shown for x=0.3 for field-cooled andzero-field-cooled conditions. Hysteresis below the freezing temperaturesignifies a spin glass.

Large MR was observed in the composition range of 0.2<x<0.4 in theseries Sr_(1-x)La_(x)Ru_(1-x)Fe_(x)O₃. As shown in FIG. 7, theresistivity decreased with the magnetic field. The MR effect is largestat x=0.3 (see also inset in FIG. 3) and is always larger at lowertemperatures (see inset), reaching 43% at 10 K in a field of 9 tesla(9T). The nearly linear field dependence indicated that MR was not yetsaturated at this field. Importantly, no MR effect was observed inferromagnetic SrRuO₃ (x=0), except in the vicinity of T_(c) (160 K), aspreviously reported (Gausephol et al., Phys. Rev. B52:3459 (1995); Kleinet al., 1999). A comparison of FIG. 6 and the MR in FIG. 7 makes itclear that, in the Sr_(1-x)La_(x)Ru_(1-x)Fe_(x)O₃ series, large MR onlyexists in the spin glass region.

To determine whether the MR effect is intrinsic or not, the above datawere compared with those of thin films that were prepared on (001)SrTiO₃ substrates using pulse laser deposition method of Choi et al.(Appl. Phys. Lett. 79:1447 (2001)). These films were strongly (001)oriented, are 100 nm thick, and have a roughness of 0.3 nm over a 2 μm×2μm area. The MR of the x=0.3 sample at 10 K is shown in FIG. 7, and itshows remarkable agreement with the data of the bulk polycrystal. Sincethese films have very few, if any, random grain boundaries, the MRobserved is a confirmed intrinsic property of the grains.

As an example of ruthenate thin films that exhibit EPIR effect at roomtemperature, a ceramic target of x=0.3 composition was prepared usingthe sol gel process. The target has a diameter of 1 inch. Using pulselaser deposition at 600° C., thin films of the same composition wasdeposited on a silicon substrate, which contains a surface platinumcoating providing a conducting bottom electrode for the thin film. Thisthin film was tested using a probe station which provides a topelectrode made of a tungsten tip. Electrical pulses of various amplitudeand duration were supplied by a pulse generator (Agilent 33250A type)and the electrical resistance of the films was measured, after theisolation of the pulse generator passage, using a multimeter with aprogrammed relay, subsequent to electrical pulse stimulation. With theapplication voltage pulses of amplitude 10 V and 600 nanoseconds (ns) induration, the resistance of the film is increased or decreased dependingon the pulse polarity as shown in FIG. 3. FIG. 4 illustrates the abilityof the material to repeatedly change resistance by the application of asingle electrical pulse of alternating polarity. The magnitude of thechange can be controlled by the pulse width and voltage.

Ca_(1-x)La_(x)Ru_(1-x)Fe_(x)O₃ series—When Example 1 was essentiallyrepeated, except using the Ca_(1-x)La_(x)Ru_(1-x)Fe_(x)O₃ series, alarge MR at x=0.3 was also observed as shown in FIG. 8. Although CaRuO₃is not ferromagnetic, a small amount of doping was found to induceferromagnetism, e.g., in Ca_(0.9)La_(0.1)Ru_(0.9)Fe_(0.1)O₃. Indeed, aspin glass region exists for 0.2<x<0.4 as shown in the inset of FIG. 8.Alloying-induced ferromagnetism is well documented in this compound, andsupports the understanding that CaRuO₃ is on the verge of bandferromagnetism (He et al., Phys. Rev. B63:172403 (2001); He et al., J.Phys.: Condens. Matter 13:8347 (2001)). As in the case of theSr_(1-x)La_(x)Ru_(1-x)Fe_(x)O₃ series in Example 1, however, the MR ofthe ferromagnetic alloy is very small (see, e.g., x=0.2 in FIG. 8). Incontrast, the large MR composition of x=0.3 is well inside the spinglass region (shown in the inset). Therefore, unlike manganites, strongferromagnetism does not coexist with strong MR in these Sr- andCa-containing perovskite ruthenates.

Sr_(2-x)Ru_(1-x)Fe_(x)O₃ series—Large MR was also observed in thelayered perovskite Sr₂RuO₄ when doped with 20% LaFeO₃. As shown in FIG.9, at 10 K and 9 T, a MR of 18% was found. This value is comparable tothe MR value of Sr_(0.8)La_(0.2)Ru_(0.8)Fe_(0.2)O₄ in FIG. 7 and muchlarger than that of Ca_(0.8)La_(0.2)Ru_(0.8)Fe_(0.2)O₃ (not a spinglass). The magnetization curve of this compound is shown in the insetof FIG. 9, which again verifies the spin glass behavior.

In all of the spin glass ruthenates studied in this example, the dcmagnetization was also found to rise with the field applied up to 9 T,with only very small hysteresis at the low field. Also, the data of dcsusceptibility at 0.01 and 1 T were almost identical. Comparing theseobservations with the nearly linear MR shown in FIGS. 7 to 9, it wasconcluded that the MR is proportional to the field-inducedmagnetization. Moreover, even without permanent magnetization, the dcmagnetization obtained at 9 T was quite large. For example, at 10 K, thedc magnetization of Sr_(0.7)La_(0.3)Ru_(0.7)Fe_(0.3)O₃ reaches 3000emu/mol, which is about 40% the saturation magnetization of SrRuO₃.Therefore, it was assumed that magnetic scattering of carriers due tospin misalignment can be substantially suppressed even in a spin glassonce a large field is applied, which is consistent with the large MRobserved;

This MR effect is consistent with some mechanism of atomic-level spinvalves. From the literature on first-principles calculations of energylevels of various ruthenium- and iron-containing perovskites (Kobayashiet al., 1998; Mazin et al., Phys. Rev. B56:2556 (1997); Mazin et al.,Phys. Rev. B61:5223 (2000); Hamada et al., in Spectroscopy of MottInsulators and Correlated Metals, (eds. Fujimori et al.), Springer,Berlin (1995) p. 95), the energy levels of Ru and Fe available forhopping electrons can be estimated and found close to each other, whichallow electron hopping in the BO₆ network. The effect of a magneticfield is to facilitate spin alignment, hence the MR effect. One wouldexpect that the spin-flip scattering increases with the concentration ofthe Ru—Fe—Ru spin valves, and that at lower temperatures alignment iseasier, so the MR is larger. In addition, in a spin glass, the fieldeffect on resistivity is gradual and linear, lacking the cusp-likefeature characteristics of the extrinsic MR.

Example 2 Solution-Polymerization Method for the Synthesis of RuthenateCompounds

Polycrystalline samples of SrRuO₃ and mixed ruthenates were prepared byconventional ceramic process and two sol-gel processes. These methodsare described below using SrRuO₃ as an example. Phase homogeneity wasverified by means of powder x-ray diffraction (XRD) and magneticmeasurements

Ceramic Process—In the ceramic process, a stoichiometric mixture ofSrCO₃ (99.99%, Alfa Aesar, Ward Hill, Mass.) and RuO₂ (99.95%, AlfaAesar) was prepared and calcined in a platinum crucible at 900° C. for12 hours. After that, powders were reground, pelletized, packed insacrificial powder of SrRuO₃, and heated at 900-1300° C. for 12-96hours, with intermediate regrinding and examining by XRD measurementsevery 12 hours to follow the progress of reaction. The XRD measurementswere conducted using CuK_(α) radiation, and the XRD patterns of theSrRuO₃ samples prepared by the ceramic route are shown in FIG. 10.

After initial calcination at 900° C. a majority perovskite phase hadalready formed, but a small amount of unreacted RuO₂ and SrO stillremained. Subsequent annealing at higher temperatures progressivelydissolved these unreacted phases. However, intermediate phases, such asSr₂RuO₄, also formed, but they disappeared after 60 hours. The reactionsequence that was used and the resulting slow kinetics are consistentwith the reported findings of other ruthenate researchers (Battle etal., 1989; Kim et al., 1995; He et al., 2001). (Notably, as reported byHe et al., 2001, KCl flux was even resorted to as a reaction aid.)

The tendency to form unintended intermediate ruthenate phases is due tothe inhomogeneous distribution of Ru and Sr, as well as the volatilityof Ru at higher temperatures. As a result, other perovskite-relatedphases with different A:B cation ratios become at least locally favored.

Solution-Polymerization (sol-gel) Processes—The solution-polymerizationmethod that was used is conceptually similar to the Pechini process(U.S. Pat. No. 3,330,697, herein incorporated by reference). Twovariations of the method were used.

Process (A)—In Process (A), a solution of Sr(NO₃)₂ was prepared bydissolving a stoichiometric amount of SrCO₃ (99.99%, Alfa Aesar) in INnitric acid, followed by the addition of an excess amount (3 time inweight) of poly-(ethylene glycol) (PEG, M_(n)˜2000 g/mol). A secondsolution of Ru (III) acetylacetonate (Ru(acac)₃) (99%, Strem Chemicals,Inc., Newburyport, Mass.) and PEG in absolute ethanol was also prepared.The two solutions were then mixed together and heated on a hot plate atT<100° C. for several hours with constant stirring to evaporate thesolvents, leaving cations that are chelated and incorporated in a moltenpolymer (T_(m)˜55° C.). The latter polymer was then slowly decomposed athigher temperature on the hot plate to obtain precursor powders. Thefinal calcination was performed in a furnace at 850° C. for 12 hours.The calcined powders were ground, pelletized and sintered attemperatures between 1200° C. and 1400° C. for 12 hours.

A disadvantage of Process (A) was the use of Ru(acac)₃ as the Ru source.Ru(acac)₃ is insoluble in water, necessitating the use of an organicsolvent, which may be incompatible with other ion precursors. Thisproblem was circumvented in Process (B) that follows, wherein RuO₂powders were used.

Process (B)—In the second sol-gel process, Process (B), a powdered RuO₂was used instead of Ru (III) acetylacetonate solution, and the amount ofPEG polymer was decreased. After solvent evaporation, the mixture wascombusted as described above, to obtain the precursor powders.

In contrast to formulation by the standard ceramic process, thesolution-polymerization methods, Process (A) and Process (B), eachprovided a uniform distribution of source cations that were chelated inthe subsequently polymerized precursor, which greatly improved theoutcome. FIG. 11 shows the two XRD patterns of SrRuO₃ samples preparedby the Process (A). After initial calcination at 850° C., only traces ofRuO₂ were detected, and they completely disappeared after a secondannealing at 1350° C. Note also, that no intermediate phases weredetected in either XRD patterns.

Selected XRD patterns of SrRuO₃ samples prepared by Process (B) are alsoshown in FIG. 11. Based upon a comparison of the production patterns ofProcess (A) and Process (B), it is clear that both yielded similarresults for SrRuO₃.

Mixed Ruthenates—As an example of mixed ruthenates,Sr_(0.7)La_(0.3)Ru_(0.7)Fe_(0.3)O₃ was prepared by the ceramic route andsynthesized by Process (B). In the ceramic process, the additionalstarting powders used were La₂O₃ (99.99%, Alfa Aesar) and Fe₂O₃ (99.99%,Alfa Aesar). In the sol-gel processes, the La and Fe sources were theirnitrate solutions. Other procedures were used as described above. Theirrespective XRD patterns are compared in FIGS. 12 and 13. The sinteredmixed ruthenate samples were additionally characterized by measuringAC/DC magnetization as a function of temperature. These measurementswere performed on a magnetometer (Quantum Design PPMS Model 6000, SanDiego, Calif.) in the temperature range of 5-300 K.

As was the case of the pure compound, SrRuO₃, the mixed ruthenatecompound, Sr_(0.7)La_(0.3)Ru_(0.7)Fe_(0.3)O₃, prepared by the ceramicroute suffered from slow reaction kinetics. For example, after 48 hoursat 1350° C., intermediate compounds still remained. Indeed, even themajority perovskite phase appeared to have non-uniform composition.Evidence of the latter came from the intensity of (100) reflection (insimple cubic notation), which should be very weak in a random solidsolution of the Sr_(0.7)La_(0.3)Ru_(0.7)Fe_(0.3)O₃ composition, butprominent in SrRuO₃ (see FIGS. 10 and 11). By comparison, Process (B)yielded a single phase material after two-step calcination as shown inFIG. 13.

The solid solution Sr_(0.7)La_(0.3)Ru_(0.7)Fe_(0.3)O₃ is a spin glassthat shows a very large magnetoresistance at low temperatures (Mamchiket al., Appl. Phys. Lett. 82:613-615 (2003)). It is known that a spinglass has a very small magnetization if it is compositionallyhomogeneous. Conversely, any magnetic impurity phases or compositionalinhomogeneities should manifest themselves in magnetic measurements,even though they may be otherwise structurally similar to the majorityphase or too small (or too few) to be detected by XRD. Therefore, thismaterial is an excellent model for comparing different processingmethods.

In FIG. 14, the DC magnetization curves of theSr_(0.7)La_(0.3)Ru_(0.7)Fe_(0.3)O₃ sample prepared by the ceramicmethod, and by the solution-polymerization Process (B), respectively,are compared. There is clearly a much larger magnetization shown in thesample prepared by the ceramic process. The extraneous magnetizationappears below 160 K, which suggests that it could come from theferromagnetic contribution of SrRuO₃ (T_(c)=160 K). The presence ofSrRuO₃ is confirmed by the weak-field AC susceptibility data (see theinset of FIG. 14) showing a ferromagnetic peak at 160 K. Notably theSrRuO₃ phase was not detected in the XRD measurement according to FIG.13, judging from the absence of (100) reflection after 48 hours at 1350°C. Therefore, it probably only exists as nano-sized clusters whose XRDreflections are obscured by line broadening and virtually undetectableby conventional XRD techniques.

In sum, compared to solid state reactions using mixed starting oxides,the solution-polymerization method significantly decreases theprocessing time and improves the compositional uniformity of theruthenate compounds as verified by XRD and magnetic measurements. Themethod especially offers a clear advantage in processing doped ruthenatecompounds, allowing magnetic properties of new ruthenates to besensitively studied without the complication of impurity phases orinhomogeneous clusters.

The disclosures of each patent, patent application and publication citedor described in this document are hereby incorporated herein byreference, in their entirety.

While the foregoing specification has been described with regard tocertain preferred embodiments, and many details have been set forth forthe purpose of illustration, it will be apparent to those skilled in theart without departing from the spirit and scope of the invention, thatthe invention may be subject to various modifications and additionalembodiments, and that certain of the details described herein can bevaried considerably without departing from the basic principles of theinvention. Such modifications and additional embodiments are alsointended to fall within the scope of the appended claims.

1. A switching device comprising: a magnetoresistive material formedfrom a ruthenate formulation of a perovskite family of materials; andplurality of contact points; wherein the magnetoresistive materialexhibits an electric-pulse-induced resistance (EPIR) switching effect.2. The device as set forth in claim 1, wherein the device is a thin filmsensor.
 3. The device as set forth in claim 1, wherein themagnetoresistive material has a magnetoresistance (MR) effect of about18% or more.
 4. The device as set forth in claim 2, wherein the EPIRswitching effect occurs at room temperature.
 5. The device as set forthin claim 2, wherein the device further comprises a ceramic target. 6.The device of claim 1, wherein the ruthenate formulation comprises anoxide formulation represented by A(Ru_(1-x)M_(x))O₃, where 0.01<x<0.5, Mis selected from the group consisting of magnetic elements; A isselected from the group consisting of K, Ca, Sr, Ba, Pb, Bi, La, Ce, Pr,Nd, Pm, Sm, Eu, Gd, Tb and Y, as well as any mixture thereof.
 7. Thedevice of claim 6, wherein the magnetic elements Mare selected from thegroup consisting of Ti, V, Cr, Mn, Fe, Co and Ni, as well as any mixturethereof.
 8. The device of claim 7, wherein elements selected from thegroup consisting of Sc, Y, Zr, Nb, Hf, Ta, Al, Ga, Ge and Sn areincorporated into M in minority to vary the magnitude of electricalresistance.
 9. The device of claim 7, wherein M is further selected fromthe group consisting of Cr, Mn, Fe, Co and Ni.
 10. The device of claim1, wherein the ruthenate formulation further comprises a spin glassformulation represented by A_(n+1)(Ru_(1-x)M_(x))_(n)O_(3n+1), where nis any positive integer, 0.01<x<0.5, M is selected from the groupconsisting of magnetic elements; A is selected from the group consistingof K, Ca, Sr, Ba, Pb, Bi, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb and Y, aswell as any mixture thereof.
 11. The device of claim 10, wherein theruthenate formulation is represented by A₂(Ru_(1-x)M_(x))O₄.
 12. Thedevice of claim 10, wherein the magnetic elements Mare selected from thegroup consisting of Ti, V, Cr, Mn, Fe, Co and Ni, as well as any mixturethereof.
 13. The device of claim 10, wherein elements selected from thegroup consisting of Sc, Y, Zr, Nb, Hf, Ta, Al, Ga, Ge and Sn areincorporated into M in minority to vary the magnitude of electricalresistance.
 14. The device of claim 12, wherein M is further selectedfrom the group consisting of Cr, Mn, Fe, Co and Ni.
 15. A method forperforming electric-pulse-induced resistance (EPIR) switchingcomprising: depositing a magnetoresistive material formed from aruthenate formulation of a perovskite family of material on a substrate,wherein the magnetoresistive material exhibits an electric-pulse-inducedresistance (EPIR) switching effect; providing a plurality of electricalcontact points on the magnetoresistive material; and creating anelectric-pulse-induced resistance switching effect by applying a fieldto the electrical contacts.
 16. The method as set forth in claim 15,wherein the magnetoresistive material has a magnetoresistance (MR)effect of about 18% or more.
 17. The method as set forth in claim 15,wherein the EPIR switching effect occurs at room temperature.
 18. Themethod of claim 15, wherein the ruthenate formulation comprises an oxideformulation represented by A(Ru_(1-x)M_(x))O₃, where 0.01<x<0.5, M isselected from the group consisting of magnetic elements; A is selectedfrom the group consisting of K, Ca, Sr, Ba, Pb, Bi, La, Ce, Pr, Nd, Pm,Sm, Eu, Gd, Tb and Y, as well as any mixture thereof.
 19. The method ofclaim 18, wherein the magnetic elements Mare selected from the groupconsisting of Ti, V, Cr, Mn, Fe, Co and Ni, as well as any mixturethereof.
 20. The method of claim 19, wherein elements selected from thegroup consisting of Sc, Y, Zr, Nb, Hf, Ta, Al, Ga, Ge and Sn areincorporated into M in minority to vary the magnitude of electricalresistance.